1. Field of the Invention
This invention relates to an ionization type smoke detector.
2. Description of the Related Art
A conventional ionization type smoke detector is disclosed, for example, in Japanese Patent Laid-Open No. 1-93895, according to which a printed circuit board is provided on a detector body through the intermediary of a shield case, and an electrode mount is provided on this printed circuit board. An inner electrode having a radiation source is attached to the electrode mount and, an intermediate electrode is secured in position above the inner electrode and is covered with an outer electrode. The printed circuit board and the outer electrode are encased in a protective cover composed of a wind shield cover, a bug screen and a cap.
Such a conventional ionization type smoke detector uses so many fixing screws that its assembling/disassembling are troublesome and time consuming.
Further, the head section of the inner electrode has a disc-like configuration, which makes the inner electrode rather difficult to handle, so that it takes a long time to secure the inner electrode in position. In order to ensure the designed performance of a detector, it is required that the electrode mount be fixed at a predetermined position on the printed circuit board. Nevertheless, it is difficult to achieve the accurate positioning with the conventional detector in which a misalignment between them is likely to occur.
In some cases, an FET is employed in a part of the electric circuit formed on the printed circuit board, which FET is electrically connected to the intermediate electrode. Conventionally, however, this FET is secured in position by soldering electrodes on both sides thereof, so that the mounting and replacement of the FET is time consuming and yet there is the fear that the FET might be damaged by heat. Further, since the FET is exposed on the printed circuit board, its inter-electrode section is subject to contamination by dust or the like, leading to reduction in insulation. Moreover, to electrically connect the electrodes of the FET mounted on the printed circuit board to the intermediate electrode, one end of a conductive member is screwed onto the intermediate electrode and the other end thereof is soldered onto the wiring pattern of the printed circuit board, or the intermediate electrode and the printed circuit board are connected to each other through the intermediary of the electrode mount by using conductive screws. Therefore, the electrical connection work is complicated and time consuming.
Further, the outer electrode is liable to move while it is being soldered on the printed circuit board, thereby making it difficult to secure it at the correct position. In addition, the outer electrode and the shield case are electrically connected indirectly to each other via the printed circuit board, which is time consuming.
When the speed at which air flows into the outer ionization chamber is excessively high, it is difficult to perform accurate smoke detection. Further, when any dust contained in the airflow adheres to a circuit part of the printed circuit board, a reduction in insulation may occur. In view of this, a structure has been devised in which the outer electrode is surrounded by a wind shield ring. However, the attaching of the wind shield ring to and the detaching of the same from the printed circuit board are time consuming. Further, the wind shield ring may be loosely positioned inside the detector.